Around 1975 David Auston discovered the so-called “Auston switch” at Bell Labs, basically the use of ultrashort laser pulses to generate an electrical or current pulse in a material that in turn, so it was discovered, radiate short broadband pulses of terahertz light. This discovery, after contributions by others in particular Dan Grischowsky and Allan Cheville (IBM Watson Research Center, around 1986) was developed into the modern laboratory technique called Terahertz Time-Domain Spectroscopy (THz-TDS). Since around 1990 this technique has been used mostly in academic laboratories for scientific research. In more recent years, starting around 2005, advances in Fiber laser systems led to a new generation of robust fiber based pulsed THz-TDS systems that are lower cost and can be deployed in the field—that is in industry, specifically in for example industrial research laboratories, medical centers or even on the manufacturing floor.
However, even though in many industries there is an expectation that there could be useful application for this technology there is at the same time in most industries as yet (2015) no proven useful applications. In the case of the semiconductor industry in particular terahertz light is expected to have great utility. Only in the terahertz region of the electromagnetic spectrum can one probe free carriers (electrons and holes) and measure the free electron spectrum directly within semiconductors, nanomaterials and thin metallic layers. Terahertz light can penetrate into and transmit through most materials, excluding metals. Since free carriers are critical to most semiconductor applications including all electronics, touch screens, LED's, batteries, power electronics and solar cells there is many sectors terahertz technology may enable highly desirable measurements.
Doping profiles play a critical role in many technologies, an important example of which is diodes. Diodes are a critical component in virtually all electronics and solar cells. The major disadvantages of the current approach are that it fails to enable the measurement of a junction doping profile using a fast, non-contact non-destructive method. The existing technologies for doping profile measurement are destructive, slow and for the most part significantly more expensive.
Currently SIMS and ECV are the techniques relied on to measure semiconductor device doping profiles. SIMS measures the chemical profile of dopants as opposed to the true doping profile (it cannot distinguish between dopant atoms at dopant sites and not at dopant sites in a crystal) and ECV measures the surface electric doping profile after etching. The shared drawbacks from these existing widely used techniques are that they are destructive measurements and have long measurement time. The art lacks an optical non-contact non-destructive measurement technique that can provide measurement of doping profiles fast and at lower cost and be incorporated into the production line for inline process monitoring with either real time or close to real time feedback.
Secondary ion mass spectroscopy (SIMS) and electrochemical capacitance-voltage (ECV) measurements represent existing alternatives and their strengths and weaknesses include the following. SIMS: destructive, time consuming, need to handle pollution after the process. Furthermore, SIMS measures the chemical doping profile as opposed to the profile of activated dopants (or free electron distribution) as do the THz technique described here. ECV: destructive, time consuming, need to handle pollution after the process.
However, the raw terahertz data is simply electrical pulses and none of this raw data can easily and directly be translated into useful information in semiconductor research or manufacturing. What the art lacks is a system and method to configure terahertz measurements and process the resultant raw data to solve a longstanding problem in semiconductor research and manufacturing (see for example Davis, K., H. Seigneur, A. Rudack, and W. V. Schoenfeld, PVMC Tackles c-Si Metrology Challenges, U.S. Photovoltaic Manufacturing Consortium. 2012). The problem solved by the present invention is the ability to accurately, rapidly and non-destructively measure the activated doping profile within (below the surface into the material) semiconductor wafers. This is done significantly more cost effectively compared with the only standard existing approach, Secondary Ion Mass Spectroscopy that is completely destructive, slow, and very expensive and cannot measure the true activated doping profile but instead only the chemical doping profile.